Wavelength locking filter

ABSTRACT

An optical system can automatically lock an adjustable spectral filter to a first wavelength of an incoming light signal, and can automatically filter an additional incoming light signal at the first wavelength. A tunable filter can have a filtering spectrum with an adjustable peak wavelength and increasing attenuation at wavelengths away from the adjustable peak wavelength. The tunable filter can receive first input light, having a first wavelength, and can spectrally filter the first input light to form first output light. A detector can detect at least a fraction of the first output light. Circuitry coupled to the detector and the tunable filter can tune the tunable filter to maximize a signal from the detector and thereby adjust the peak wavelength to match the first wavelength. The tunable filter further can receive second input light and spectrally filter the second input light at the first wavelength.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. patent application Ser. No.15/079,590, filed Mar. 24, 2016, which claims the benefit of U.S.Provisional Application No. 62/137,982, filed on Mar. 25, 2015, all ofwhich are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The subject matter disclosed herein generally relates to spectralfiltering of optical signals. Specifically, the present disclosureaddresses automatically locking an adjustable spectral filter to a firstwavelength of an incoming light signal, and automatically filtering anadditional incoming light signal at the first wavelength.

BACKGROUND

In some technical fields, such as telecommunications, it can bedesirable to spectrally filter one light signal to match a wavelength ofanother light signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures of the accompanying drawings provide non-limiting andnon-exhaustive examples of some embodiments. Like reference numeralsrefer to like parts throughout the various view unless otherwisespecified. The following figures are not drawn to scale.

FIG. 1 is a block diagram showing an example of an optical system thatcan automatically spectrally filter one light signal to match awavelength of another light signal, in accordance with some embodiments.

FIG. 2 is a block diagram showing a specific configuration of theoptical system of FIG. 1, including an optical ring resonator, a powertap, a detector, circuitry, and a heater, in accordance with someembodiments.

FIG. 3 is a block diagram showing an alternate configuration of theoptical system of FIG. 1, including multiple optical ring resonatorsarranged in series, in accordance with some embodiments.

FIG. 4 is a block diagram showing another example of an optical systemthat can automatically spectrally filter multiple light signals to matcha wavelength of another light signal, optionally with multiplexers anddemultiplexers on its inputs and outputs, respectively, in accordancewith some embodiments.

FIG. 5 is a block diagram showing a specific configuration of an opticalsystem, including multiple optical ring resonators, in accordance withsome embodiments.

FIG. 6 is a flow chart showing an example of a method for automaticallyspectrally filtering one light signal to match a wavelength of anotherlight signal, in accordance with some embodiments.

FIG. 7 is a block diagram showing another example of an optical system,in accordance with some embodiments.

DETAILED DESCRIPTION

An optical system can automatically lock an adjustable spectral filterto a first wavelength of an incoming light signal, and can automaticallyfilter an additional incoming light signal at the first wavelength. Insome examples, a tunable source can automatically lock to the spectralfilter, and produce light at the first wavelength. Such an opticalsystem can be simpler than a comparable system that uses a detectionsystem to sense the first wavelength of the incoming light signal, and aseparate tuning system to actively tune a downstream spectral filter ora tunable source to match the first wavelength.

FIG. 1 is a block diagram showing an example of an optical system 100that can automatically spectrally filter one light signal to match awavelength of another light signal, in accordance with some embodiments.The configuration of FIG. 1 is but one example of an optical system 100that can automatically spectrally filter one light signal to match awavelength of another light signal; other suitable configurations canalso be used.

A tunable filter 102 can have a filtering spectrum with an adjustablepeak wavelength and increasing attenuation at wavelengths away from theadjustable peak wavelength. FIG. 1 shows an example of a filteringspectrum for the tunable filter 102, in which a power transmission curveappears as a peak at an adjustable peak wavelength, and decreasing powertransmission (e.g., increasing attenuation) away from the adjustablepeak wavelength. The power transmission curve can include sloping sideson either side of the peak. The peak wavelength can be locked to awavelength of a light signal, as explained below, so that the wholepower transmission curve (e.g., the peak wavelength and the slopingsides) can shift upward and downward in wavelength in real time, asneeded.

The tunable filter 102 can receive first input light 104, having a firstwavelength denoted as λ1 in FIG. 1. The first wavelength λ1 can varyover time. The tunable filter 102 can spectrally filter the first inputlight 104 to form first output light 106. The optical system 100 can beconfigured so that the tunable filter 102 automatically follows thefirst wavelength λ1. As a result, the first output light 106 can have awavelength that matches that of the first input light 104, namely λ1.

A power tap 108 can direct a fraction of the first output light 106 to adetector 110. The fraction is between 0% and 100%, and can be on theorder of 0.1%, 0.5%, 1%, 2%, 5%, 10%, or 20%, among other values. Ingeneral, the fraction should be large enough so that the detector andcircuitry can function with a sufficiently large signal-to-noise ratio,but small enough so that the remaining first output light 106 canperform its intended task, such as providing data to a particulartelecommunications channel. In some examples, the fraction can be 100%,where all of the first output light 106 is directed onto the detector110. For these examples, the power tap 108 can be absent, or can be acoupling between a waveguide and the detector 110.

Circuitry 112 coupled to the detector 110 and the tunable filter 102 cantune the tunable filter 102 to maximize a signal from the detector 110and thereby adjust the peak wavelength to match the first wavelength λ1.The circuitry 112 can be processor-based, can be formed from discretecomponents, or can be a combination of processor-based and discrete. Insome examples, the circuitry 112 can apply a dither to the tunablewavelength, so that the wavelength of the output light 106 varies with aperiodic oscillation. In these examples, the circuitry 112 can sense apolarity of the periodic oscillation, can use the polarity to determinewhich side of the peak wavelength the output light 106 is on, and canform a servo that locks the peak wavelength of the tunable filter 102 tothe first wavelength λ1 of the first input light 104. In other examples,the circuitry 112 can dither the peak wavelength of the tunable filter102. In still other examples, the circuitry 112 can apply ahill-climbing algorithm to set the peak wavelength to match the firstwavelength λ1.

The tunable filter 102 can further receive second input light 114, andcan spectrally filter the second input light 114 to form second outputlight 116. The second output light can be spectrally filtered with apeak at the first wavelength λ1 and increasing attenuation atwavelengths away from the first wavelength λ1. In FIG. 1 and subsequentfigures, the second input light 114 (before filtering) has a wavelengthor wavelength range denoted by λ′, and the second output light 116(after filtering) has a wavelength denoted by “λ′ filtered to leave onlyλ1”.

FIG. 2 is a block diagram showing a specific configuration of theoptical system of FIG. 1, including an optical ring resonator, a powertap, a detector, circuitry, and a heater, in accordance with someembodiments. The configuration of FIG. 2 is but one example; othersuitable configurations can also be used.

In the optical system 200 of FIG. 2, the tunable filter is an opticalring resonator 202 formed from at least one waveguide 204 arranged in aclosed path. The optical ring resonator 202 can propagate light in afirst direction 206 around the closed path (e.g. clockwise in FIG. 2)and a second direction 208, opposite the first direction 206 (e.g.,counterclockwise in FIG. 2), around the closed path.

The resonance of an optical ring resonator, such as 202, is a functionof an optical path length around the optical ring resonator. Theresonator shows relatively high resonance for optical path lengths thatare an integral number (e.g., an integer-valued number) of wavelengths,and relatively low resonance for optical path lengths away from theintegral number of wavelengths. In other words, a given optical ringresonator shows resonance at wavelengths for which an integral number“fit” within the optical path of the optical ring resonator. The spacingbetween adjacent resonant wavelengths is referred to as a free spectralrange. In some examples, the free spectral range can be greater than aspecified range of wavelengths for the first input light 104. Forexample, the specified range of wavelength can correspond to a range ofwavelengths of a channel in a telecommunications system, or a specifiedrange of wavelengths within a particular channel.

A first input waveguide 212 can inject the first input light 104, havinga first wavelength λ1, into the optical ring resonator 202 in the firstdirection 206. A first output waveguide 214 can extract the first outputlight 106 from the optical ring resonator 202 in the first direction206. A second input waveguide 222 can inject the second input light 114into the optical ring resonator 202 in the second direction 208. Asecond output waveguide 224 can extract the second output light 116 fromthe optical ring resonator 202 in the second direction 208. In someexamples, at least one of the input or output waveguides is a discretewaveguide. In some examples, the input and/or output waveguides areconstructed as a single waveguide with a coupling region parallel to ormerging with the waveguide 204 of the optical ring resonator 202. Thepower tap 108 can extract a portion of the first output light 106 fromthe first output waveguide 214. The detector 110 and circuitry 112function the same as the corresponding elements shown in FIG. 1.

In the example of FIG. 2, the tunable filter can further include amaterial having a temperature-dependent refractive index disposed in anoptical path of the optical ring resonator 202. In the example of FIG.2, the tunable filter can further include a heater 210 that cancontrollably heat at least a portion of the material, and thereby changean optical path length around the optical ring resonator and therebychange a resonant wavelength of the optical ring resonator. In someexamples, the tunable filter can further include multiple heatersdisposed around the optical ring resonator 202, all of which are coupledto the circuitry 112. The circuitry 112 can further tune the tunablefilter by heating the portion of the material.

In other configurations, the tunable filter can include other ways toadjust the optical path length of the optical ring resonator 202. Forexample, the tunable filter can use a carrier injection, such as from aforward-biased PIN diode, to induce a change in refractive index viafree-carrier absorption from a material disposed in the waveguide of theoptical ring resonator 202. Carrier injection is especially well-suitedfor III-V semiconductor materials. Other suitable ways to adjust theoptical path length can also be used.

In some examples, some or all of the elements of FIG. 2 are formed atthe wafer level, rather than as discrete components that are assembledafter they have been manufactured. In some examples, a top silicon layerof a silicon-on-insulator wafer can shaped to define the optical ringresonator 202, the first and second input waveguides 212, 222, and thefirst and second output waveguides 214, 224. Other suitable substratematerials can also be used. In some examples, additional material orelements can be deposited or grown on the substrate. For example, thematerial having a temperature-dependent refractive index can bedeposited or grown in the waveguide 204 of the optical ring resonator202.

FIG. 3 is a block diagram showing an alternate configuration of theoptical system of FIG. 1, including multiple optical ring resonatorsarranged in series, in accordance with some embodiments. Optical system300 can include a plurality of optical ring resonators, each of theplurality of optical ring resonators being formed from at least onewaveguide arranged in a respective closed path. The configuration ofFIG. 3 is but one example; other suitable configurations can also beused.

Compared with the optical system 200 of FIG. 2, which includes a singleoptical ring resonator 202, the optical system 300 of FIG. 3 includes afirst optical ring resonator 302 and a second optical ring resonator 304connected in series between the input waveguides 212, 222 and the outputwaveguides 214, 224. Cascading one or more optical ring resonators inseries can increase the free spectral range of the combination ofresonators, which can be desirable in some cases. In some examples,cascading the optical ring resonators in series, and thereby increasingthe free spectral range of the combination of resonators, can bereferred to as a Vernier approach. In some configurations, more than twooptical ring resonators can be cascaded in series. In otherconfigurations, two or more optical ring resonators can be positioned inparallel between the input waveguides 212, 222 and the output waveguides214, 224. The power tap, detector, circuitry, and heater are omittedfrom FIG. 3 for clarity, and can function in a manner similar to FIG. 2.

FIG. 4 is a block diagram showing another example of an optical system400 that can automatically spectrally filter multiple light signals tomatch a wavelength of another light signal, optionally with multiplexersand demultiplexers on its inputs and outputs, respectively, inaccordance with some embodiments. The configuration of FIG. 4 is but oneexample; other suitable configurations can also be used.

In some examples, the tunable filter 402 can further receive third inputlight and spectrally filter the third input light to form third outputlight, the third output light being spectrally filtered with a peak atthe first wavelength and increasing attenuation at wavelengths away fromthe first wavelength. The tunable filter 402 can further extend thespectral filtering to fourth, fifth, and more than fifth input/outputs.In some examples, the tunable filter can automatically tune a pluralityof input lights to the wavelength of a particular input light.

In some examples, optical system 400 can further include a first inputmultiplexer 404 configured to multiplex a plurality of inputs 406 intothe first input light 408. In some of these examples, optical system 400can further include a first output demultiplexer 410 configured todemultiplex a plurality of outputs 412 from the first output light 414.In some examples, optical system 400 can further include a second inputmultiplexer 416 configured to multiplex a plurality of inputs 418 intothe second input light 420. In some of these examples, optical system400 can further include a second output demultiplexer 422 configured todemultiplex a plurality of outputs 424 from the second output light 426.Any or all of these multiplexers or demultiplexers can use at least oneof frequency-division multiplexing, time-division multiplexing,polarization-division multiplexing. In addition, any or all could useelectrical modulation or electrical demodulation.

Similarly, in some examples, optical system 400 can further include athird input multiplexer 428 configured to multiplex a plurality ofinputs 430 into the third input light 432. In some of these examples,optical system 400 can further include a third output demultiplexer 434configured to demultiplex a plurality of outputs 436 from the thirdoutput light 438.

The multiplexers and demultiplexers are optional, and can applied to anyor all of the inputs and/or outputs to the tunable filter 402.

FIG. 5 is a block diagram showing a specific configuration of an opticalsystem 500, including multiple optical ring resonators 502, 504, inaccordance with some embodiments. The configuration of FIG. 5 is but oneexample; other suitable configurations can also be used.

In the optical system 500, the optical ring resonators 502, 504 areincorporated into a network in a manner that blurs the distinctionbetween purely input waveguides and purely output waveguides. As aresult, the waveguides that provide the input light and output light arereferred to simply as waveguides. As with the examples discussed above,two or more waveguides can be constructed as a single waveguide thatcontacts a ring resonator at a coupling region. The followingdescription uses the terms left, right, top, bottom, clockwise andcounter-clockwise only for convenience, with respect to the orientationsshown in FIG. 5.

Waveguide 506 extends to the right and provides first input light 104 toa top of a first optical ring resonator 502 in a clockwise direction.Waveguide 512 extends to the right from the top of the first opticalring resonator 502. Waveguide 508 extends to the left from the bottom ofthe first optical ring resonator 502. Waveguide 510 extends to the leftfrom a split in waveguide 508 to provide a first output light 522.Waveguide 512 loops downward to join waveguide 516 and extend to theleft along waveguide 514. Waveguide 514 extends to the left to a top ofsecond optical ring resonator 502 in a counter-clockwise direction.Second output light 526 extends to the left from the top of the secondoptical ring resonator 502. First and second output light 522, 526 areat the first wavelength, of the first input light 104. Waveguide 520extends to the left and provides second input light 114 to a bottom ofthe second optical ring resonator 504 in a clockwise direction.Waveguide 518 extends to the left from the bottom of the second opticalring resonator 504. Third output light 524 extends to the right from thebottom of the first optical ring resonator 502. Fourth output light 516extends to the right from the top of the second optical ring resonator504. Third and fourth output light 524, 528 are spectrally filtered tomatch the first wavelength, of the first input light 104. Each splittingor joining of two waveguides can have a suitable splitting ratio (e.g.,the ratio can be 50%, or another suitable value).

In some examples, optical ring resonators 502 and 504 can have differentsizes, and, therefore different optical path lengths. As such, thedifferently-sized optical ring resonators 502 and 504 can form a Vernierfilter. For example, to tune the filters to match wavelength λ1, theoptical system 500 can tune the first optical ring resonator 502 tomaximize the first output light 522. Next, the optical system 500 cantune the second optical ring resonator 504 to maximize the second outputlight 526. Next, for examples in which a source of the second inputlight 114 is tunable, the optical system 500 can tune the source tomaximize the third output light 524. This is but one example; othersuitable examples can also be used.

The configuration of FIG. 5 provides two outputs for each input. Otherconfigurations can provide more than two outputs for each input.

FIG. 6 is a flow chart showing an example of a method 600 forautomatically spectrally filtering one light signal to match awavelength of another light signal, in accordance with some embodiments.The method 600 can be executed by any suitable optical system with atunable filter, such as 100, 200, 300, 400, 500, or another suitableoptical system. The method 600 is but one example of a method 600 forautomatically spectrally filtering one light signal to match awavelength of another light signal; other suitable methods can also beused.

At operation 602, the optical system can receive first input light,having a first wavelength, at a tunable filter. The tunable filter canhave a filtering spectrum with an adjustable peak wavelength andincreasing attenuation at wavelengths away from the adjustable peakwavelength.

At operation 604, the optical system can spectrally filter the firstinput light with the tunable filter to form first output light.

At operation 606, the optical system can detect a fraction of the firstoutput light.

At operation 608, the optical system can tune the tunable filter tomaximize the detected fraction of the first output light, therebyadjusting the peak wavelength to match the first wavelength. In someexamples, tuning the tunable filter to maximize the detected fraction ofthe first output light can include: heating at least a portion of amaterial having a temperature-dependent refractive index and positionedin an optical path of an optical ring resonator.

At operation 610, the optical system can receive second input light atthe tunable filter.

At operation 612, the optical system can spectrally filter the secondinput light to form second output light, the second output light beingspectrally filtered with a peak at the first wavelength and increasingattenuation at wavelengths away from the first wavelength.

In some examples, the method 600 can optionally further includemultiplexing a plurality of inputs into the first input light; anddemultiplexing a plurality of outputs from the first output light.

In some examples, the method 600 can optionally further includemultiplexing a plurality of inputs into the second input light; anddemultiplexing a plurality of outputs from the second output light.

Thus far, the description of the optical system and method havediscussed receiving a light input at a first wavelength, tuning thetunable filter to match the first wavelength, and tuning a second lightinput with the tunable filter to leave only the first wavelength. Insome examples, the optical system and method can further wavelength-tunethe optical source that produces the second light input to maximizelight passed through the tunable filter.

FIG. 7 is a block diagram showing another example of an optical system700, in accordance with some embodiments. The optical system 700includes a tunable optical source 714, such as a tunable laser, whichproduces the second light input 114. In some examples, a power tap (notshown) can direct a fraction of light from the tunable optical source714 to form the second light input 114, and direct a complementaryfraction of light from the tunable optical source 714 to anotherlocation. Other suitable optical sources can also be used. The opticalsource can be positioned upstream to the second input waveguide 222, andcan produce the second light input 114. In addition, the optical system700 can include an additional optical tap 708 to direct a fraction ofthe second light output 116 onto an additional detector 710.Wavelength-tuning circuitry 712 can adjust the wavelength of the opticalsource to maximize the signal from the additional detector 710, andthereby tune the wavelength of the second input light 114 to match thepeak wavelength of the optical ring resonator 202, which in turn istuned to match the wavelength of the first input light 104. Thewavelength-tuning circuitry 712 can be made integral with circuitry 112or can be separate from circuitry 112. In the configuration of FIG. 7,other suitable ways to vary the optical path length of the optical ringresonator 202 can also be used.

Additionally, thus far, the optical ring resonators have been used tofilter counter-propagating signals. For example, in FIG. 2, one signalpropagates around the optical ring resonator 202 in a first direction206, while a second signal propagates around the optical ring resonator202 in a first direction 208. As an alternative configuration, bothsignals can propagate in the same direction. In this alternativeconfiguration, the second input and output waveguides 222, 224 areswapped. In addition, to generate the servo signals that can drive theheater 210 (or drive another suitable tuning mechanism), the inputs canbe modulated, so that only one of the first 104 or second 114 lightinputs are on at any given time.

The preceding description includes discussion of figures havingillustrations given by way of example of implementations of embodimentsof the disclosure. The drawings should be understood by way of example,and not by way of limitation. As used herein, references to one or more“embodiments” are to be understood as describing a particular feature,structure, or characteristic included in at least one implementation ofthe disclosure. Thus, phrases such as “in one embodiment” or “in analternate embodiment” appearing herein describe various embodiments andimplementations of the disclosure, and do not necessarily all refer tothe same embodiment. However, they are also not necessarily mutuallyexclusive.

Throughout this specification, several terms of art are used. Theseterms are to take on their ordinary meaning in the art from which theycome, unless specifically defined herein or the context of their usewould clearly suggest otherwise. In the preceding description numerousspecific details are set forth to provide a thorough understanding ofthe embodiments. One skilled in the relevant art will recognize,however, that the techniques described herein can be practiced withoutone or more of the specific details, or with other methods, components,materials, etc. In other instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringcertain aspects.

Reference throughout the foregoing specification to “one embodiment” or“an embodiment” means that a particular feature, structure orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present disclosure. Thus, appearancesof the phrases “in one embodiment” or “in an embodiment” in variousplaces throughout the specification are not necessarily all referring tothe same embodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments. In addition, it is appreciated that the figures providedare for explanation purposes to persons ordinarily skilled in the artand that the drawings are not necessarily drawn to scale. It is to beunderstood that the various regions, layers and structures of figuresmay vary in size and dimensions.

The above described embodiments of the disclosure may include discretedevices, or may be components of a photonic integrated circuit (PIC).PICs that consist of multiple photonic components offer many advantagesover those that consist of discrete photonic devices, such as higherefficiency due to the removal of coupling losses between components,fewer packages and packaging steps, smaller size, and overall betterperformance.

The above-described embodiments of the disclosure may include SOI orsilicon based (e.g., silicon nitride (SiN)) devices, or may includedevices formed from both silicon and a non-silicon material. Saidnon-silicon material (alternatively referred to as “heterogeneousmaterial”) may include one of III-V material, magneto-optic material, orcrystal substrate material.

III-V semiconductors have elements that are found in group III and groupV of the periodic table (e.g., Indium Gallium Arsenide Phosphide(InGaAsP), Gallium Indium Arsenide Nitride (GaInAsN)). The carrierdispersion effects of III-V based materials may be significantly higherthan in silicon based materials, as electron speed in III-Vsemiconductors is much faster than that in silicon. In addition, III-Vmaterials have a direct bandgap which enables efficient creation oflight from electrical pumping. Thus, III-V semiconductor materialsenable photonic operations with an increased efficiency over silicon forboth generating light and modulating the refractive index of light.

Thus, III-V semiconductor materials enable photonic operation with anincreased efficiency at generating light from electricity and convertinglight back into electricity. The low optical loss and high qualityoxides of silicon are thus combined with the electro-optic efficiency ofIII-V semiconductors in the heterogeneous optical devices describedbelow. In embodiments of the disclosure, said heterogeneous devicesutilize low loss heterogeneous optical waveguide transitions between thedevices' heterogeneous and silicon-only waveguides.

Magneto-optic materials allow heterogeneous PICs to operate based on themagneto-optic (MO) effect. Such devices may utilize the Faraday Effect,in which the magnetic field associated with an electrical signalmodulates an optical beam, offering high bandwidth modulation, androtates the electric field of the optical mode enabling opticalisolators. Said magneto-optic materials may include, for example,materials such as such as iron, cobalt, or yttrium iron garnet (YIG).

Crystal substrate materials provide heterogeneous PICs with a highelectro-mechanical coupling, linear electro optic coefficient, lowtransmission loss, and stable physical and chemical properties. Saidcrystal substrate materials may include, for example, lithium niobate(LiNbO₃) or lithium tantalate (LiTaO₃).

In the foregoing detailed description, the method and apparatus of thepresent disclosure have been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present disclosure. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. An optical filter, comprising: a first opticalring resonator; a first input waveguide configured to deliver firstlight to the first optical ring resonator to propagate in a firstdirection around the first optical ring resonator; a second optical ringresonator; a first waveguide segment configured to deliver the firstlight from the first optical ring resonator to the second optical ringresonator to propagate in a second direction around the second opticalring resonator; a first output waveguide configured to propagate thefirst light away from the second optical ring resonator, the first lightin the first output waveguide being spectrally filtered with a filteringspectrum that includes a peak wavelength and increasing attenuation atwavelengths away from the peak wavelength; a second input waveguideconfigured to deliver second light to the second optical ring resonatorto propagate in a direction opposite the second direction around thesecond optical ring resonator; a second waveguide segment configured todeliver the second light from the second optical ring resonator to thefirst optical ring resonator to propagate in a direction opposite thefirst direction around the first optical ring resonator; and a secondoutput waveguide configured to propagate the second light away from thefirst optical ring resonator, the second light in the second outputwaveguide being spectrally filtered with a filtering spectrum thatincludes the peak wavelength and increasing attenuation at wavelengthsaway from the peak wavelength.
 2. The optical filter of claim 1,wherein: the first and second optical ring resonators are tunable; andthe peak wavelength is adjustable, such that tuning at least one of thefirst and second optical ring resonators adjusts the peak wavelength. 3.The optical system of claim 2, further comprising: a first tapconfigured to direct a fraction of the first light out of the firstwaveguide segment; a first detector configured to detect at least aportion of the first light from the first tap; and first circuitryconfigured to tune the first optical ring resonator in response to asignal from the first detector.
 4. The optical system of claim 3,wherein the first circuitry is configured to maximize a signal from thefirst detector to thereby tune a resonant wavelength of the firstoptical ring resonator to match a wavelength of the first light.
 5. Theoptical system of claim 3, further comprising: a material having atemperature-dependent refractive index disposed in an optical path ofthe first optical ring resonator; and a heater configured tocontrollably heat at least a portion of the material, wherein the firstcircuitry is configured to tune the first optical ring resonator byselectively heating the portion of the material to thereby change anoptical path length around the first optical ring resonator and therebychange a resonant wavelength of the first optical ring resonator.
 6. Theoptical system of claim 3, further comprising: a material having acurrent-dependent refractive index via free-carrier absorption disposedin an optical path of the first optical ring resonator; and a firstforward-biased PIN diode configured to controllably inject current intoat least a portion of the material, wherein the first circuitry isconfigured to tune the first optical ring resonator by selectivelyinjecting the current into the portion of the material to thereby changean optical path length around the first optical ring resonator andthereby change a resonant wavelength of the first optical ringresonator.
 7. The optical system of claim 3, further comprising: asecond tap configured to direct a fraction of the second light out ofthe second waveguide segment; a second detector configured to detect atleast a portion of the second light from the second tap; and secondcircuitry configured to tune the second optical ring resonator inresponse to a signal from the second detector.
 8. The optical system ofclaim 7, wherein the second circuitry is configured to maximize a signalfrom the second detector to thereby tune a resonant wavelength of thesecond optical ring resonator to match a wavelength of the second light.9. The optical system of claim 1, wherein the first optical ringresonator and the second optical ring resonator, together, determine thepeak wavelength such that an optical path length around the firstoptical ring resonator is an integral multiple of the peak wavelength,and an optical path length around the second optical ring resonator isan integral multiple of the peak wavelength.
 10. The optical system ofclaim 9, wherein: the optical path length around the first optical ringresonator differs from the optical path length around the second opticalring resonator; and the first optical ring resonator and the secondoptical ring resonator, together, determine the adjustable peakwavelength via the Vernier effect.
 11. The optical system of claim 9,wherein the optical path length around the first optical ring resonatoris the same as the optical path length around the second optical ringresonator.
 12. The optical system of claim 1, wherein the first inputwaveguide, the first optical ring resonator, the first waveguidesegment, the second optical ring resonator, the first output waveguide,the second input waveguide, the second waveguide segment, and the secondoutput waveguide are all positioned in a plane.
 13. The optical systemof claim 12, wherein: the second input waveguide is configured todeliver the second light to the second optical ring resonator topropagate in a third direction opposite the second direction around thesecond optical ring resonator; the second waveguide segment isconfigured to deliver the second light from the second optical ringresonator to the first optical ring resonator to propagate in a fourthdirection opposite the first direction around the first optical ringresonator; the first and third directions are clockwise; and the secondand fourth directions are counter-clockwise.
 14. The optical system ofclaim 12, wherein: the second input waveguide is configured to deliverthe second light to the second optical ring resonator to propagate in athird direction opposite the second direction around the second opticalring resonator; the second waveguide segment is configured to deliverthe second light from the second optical ring resonator to the firstoptical ring resonator to propagate in a fourth direction opposite thefirst direction around the first optical ring resonator; the first andthird directions are counter-clockwise; and the second and fourthdirections are clockwise.
 15. A method, comprising: delivering firstlight from a first input waveguide to a first optical ring resonator;propagating the first light in a first direction around the firstoptical ring resonator; delivering the first light via a first waveguidesegment from the first optical ring resonator to a second optical ringresonator; propagating the first light in a second direction around thesecond optical ring resonator; propagating the first light via a firstoutput waveguide away from the second optical ring resonator, the firstlight in the first output waveguide being spectrally filtered with afiltering spectrum that includes a peak wavelength and increasingattenuation at wavelengths away from the peak wavelength; deliveringsecond light via a second input waveguide to the second optical ringresonator; propagating the second light in a direction opposite thesecond direction around the second optical ring resonator; deliveringthe second light via a second waveguide segment from the second opticalring resonator to the first optical ring resonator; propagating thesecond light in a direction opposite the first direction around thefirst optical ring resonator; and propagating the second light via asecond output waveguide away from the first optical ring resonator, thesecond light in the second output waveguide being spectrally filteredwith a filtering spectrum that includes the peak wavelength andincreasing attenuation at wavelengths away from the peak wavelength. 16.The method of claim 15, further comprising: tuning at least one of thefirst and second optical ring resonators to adjust the peak wavelength.17. The method of claim 16, further comprising: controllably heating atleast a portion of material having a temperature-dependent refractiveindex disposed in an optical path of the first optical ring resonator,thereby changing an optical path length around the first optical ringresonator and thereby changing a resonant wavelength of the firstoptical ring resonator.
 18. The method of claim 16, further comprising:injecting current into at least a portion of material having acurrent-dependent refractive index via free-carrier absorption disposedin an optical path of the first optical ring resonator, thereby changingan optical path length around the first optical ring resonator andthereby changing a resonant wavelength of the first optical ringresonator.
 19. The method of claim 16, further comprising: detecting afraction of the first light from the first waveguide segment; and tuningthe first optical ring resonator to maximize the detected fraction ofthe first light, to thereby tune a resonant wavelength of the firstoptical ring resonator to match a wavelength of the first light.
 20. Anoptical filter, comprising: a first tunable optical ring resonator; afirst input waveguide configured to deliver first light to the firsttunable optical ring resonator to propagate in a first direction aroundthe first tunable optical ring resonator; a second tunable optical ringresonator; a first waveguide segment configured to deliver the firstlight from the first tunable optical ring resonator to the secondtunable optical ring resonator to propagate in a second direction aroundthe second tunable optical ring resonator; a first output waveguideconfigured to propagate the first light away from the second tunableoptical ring resonator, the first light in the first output waveguidebeing spectrally filtered with a filtering spectrum that includes a peakwavelength and increasing attenuation at wavelengths away from the peakwavelength, wherein the tunable first optical ring resonator and thesecond tunable optical ring resonator, together, determine the peakwavelength such that an optical path length around the first tunableoptical ring resonator is an integral multiple of the peak wavelength,and an optical path length around the second tunable optical ringresonator is an integral multiple of the peak wavelength, wherein theoptical path length around the first optical ring resonator differs fromthe optical path length around the second optical ring resonator,wherein the first optical ring resonator and the second optical ringresonator, together, determine the adjustable peak wavelength via theVernier effect; a second input waveguide configured to deliver secondlight to the second tunable optical ring resonator to propagate in adirection opposite the second direction around the second tunableoptical ring resonator; a second waveguide segment configured to deliverthe second light from the second tunable optical ring resonator to thefirst tunable optical ring resonator to propagate in a directionopposite the first direction around the first tunable optical ringresonator; and a second output waveguide configured to propagate thesecond light away from the first tunable optical ring resonator, thesecond light in the second output waveguide being spectrally filteredwith a filtering spectrum that includes the peak wavelength andincreasing attenuation at wavelengths away from the peak wavelength;wherein the first input waveguide, the first tunable optical ringresonator, the first waveguide segment, the second tunable optical ringresonator, the first output waveguide, the second input waveguide, thesecond waveguide segment, and the second output waveguide are allpositioned in a plane.